Light-emitting elements (organic EL elements) including organic compounds and utilizing electroluminescence (EL) have been put to more practical use. In the basic structure of such a light-emitting element, an organic compound layer containing a light-emitting material (an electroluminescent (EL) layer) is provided between a pair of electrodes. Carriers are injected by application of voltage to the element, and light emission can be obtained from the light-emitting material by using the recombination energy of the carriers.
The light-emitting elements are self-luminous elements and thus have advantages such as high visibility and no need for backlight when used as pixels of a display, and are suitable as flat panel display elements. In addition, it is also a great advantage that a display including such light-emitting elements can be manufactured as a thin and lightweight display. Furthermore, an extremely high response speed is also a feature thereof.
In such light-emitting elements, light-emitting layers can be successively formed two-dimensionally, so that planar light emission can be obtained. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Furthermore, light emission from an organic compound can be light emission which does not include UV light by selecting a material; thus, light-emitting elements also have great potential as planar light sources used in lighting devices and the like.
Displays or lighting devices including light-emitting elements can be suitably used for a variety of electronic devices as described above; thus, research and development of light-emitting elements have progressed for higher efficiency or longer element lifetimes. In particular, an organic compound is mainly used in an EL layer and greatly affects an improvement in the characteristics of the light-emitting element. For this reason, a variety of novel organic compounds have been developed.
The lifetime and properties of a light-emitting element including an organic compound are greatly affected by the properties of a hole-transport material in some cases. In particular, the lifetime and properties of a light-emitting element differ significantly according to the type of hole-transport material.
In general, a compound in which π conjugated systems spread across a molecule, which is typified by an aromatic compound, is used as a hole-transport material. In particular, an aromatic amine compound has been developed. The properties of a hole-transport material in an aromatic amine compound greatly depend on an aromatic skeleton.
Although aromatic amine compounds including various aromatic skeletons have been reported and the properties and reliability of light-emitting elements including the compounds have been improved, advanced requirements for various properties including efficiency and durability are not yet satisfied (Patent Document 1, for example).